RECENT EVENTS IN RELATIVITY — ROTHMAN 387 



and more elaborate General Theory (1915) deals with accelerated 

 systems. In an intermediate paper of 1911, Einstein discussed the 

 Principle of Equivalence (table 1) , which has been the subject of some 

 of the recent experiments. 



This principle, in effect, says that if we do an experiment on the 

 surface of the earth, under 1 g. of gravitational acceleration, then 

 we should get exactly the same result if we do the experiment out in 

 space, in a ship undergoing 1 g. of rocket acceleration. In fact, we 

 see here that the terminology of the astronaut explicitly recognizes 

 the Principle of Equivalence, for 1 g. of acceleration always means 

 the same thing, whether it is caused by gravity or rocket thrust. 



What this implies is a very basic assumption : Gravitational mass 

 (the mass which determines the force of gravity) is exactly the same 

 as inertial mass (the mass which determines the acceleration resulting 

 from an applied force) . 



If this were not true, then bodies with different masses would fall 

 at different rates. It has not always been obvious to people that 

 different masses do fall at the same rate. Since the time of Galileo 

 we have believed this assumption to be true. Nevertheless, our experi- 

 ments are only approximations, and there is always the chance that 

 an experiment giving another decimal place of accuracy might dis- 

 cover small differences between gravitational and inertial mass. Be- 

 cause of this, we are always on the alert for new and novel experi- 

 ments which tend to settle the question more definitely. 



The predictions of the Special Theory, listed in table 2, have been 

 verified by numerous experiments during the past 50 years. How- 

 ever, the Principle of Equivalence has not been so fortunate, since the 

 effects which it predicts are so minute that until very recently labora- 

 tory experiments of the required precision have been out of the 

 question. 



One effect which the Principle of Equivalence predicts is the "gravi- 

 tational red shift." If a source of light emits radiation which travels 

 from a region of low gravitational potential to a region of high 

 gravitational potential — that is, if the light is traveling up, then the 



Table 2. — Results of the Special Theory of Relativity 



1. A body which is moving relative to an observer appears 

 to be shortened in the direction of motion. 



2. The mass of this moving body is greater than when it is 

 at rest. This increase of mass is directly related to the 

 kinetic energy of the moving body. 



3. The total mass of a body is related to its total energy 

 according to the expression : E=mc*. 



4. A clock which is moving relative to an observer runs 

 more slowly than a clock at rest. 



5. The maximum velocity for the transmission of any 

 signal is the speed of light. 



